Transistors in monolithic semiconductor devices have relatively high collector resistances, and are thus not well suited for high power devices, e.g. greater than 1 amp devices. Such known transistors comprise, for example, a P base diffusion in an N type epitaxial layer, the N epitaxial layer being grown on a P type substrate. An N+ buried layer is formed by diffusion into the P substrate before the growth of the N type epitaxial layer and below the base region. An N+ emitter diffusion is formed in the P base diffusion region and a collector connection region is formed by an N+ diffusion in the N type epitaxial layer laterally displaced from the P type base diffusion region.
The collector resistance for such a transistor is formed by the path between the lower edge of the P base region and the upper edge of the N+ buried layer, plus the lateral path from under the base region to under the N+ collector connection region mainly via the N+ buried layer region, and plus the vertical path from the N+ buried layer up to the collector connection region. For large well designed geometries the lateral path accounts for about half of the total collector resistance and the vertical path beneath the collector connection diffusion region accounts for about 40 percent of the total.
One known technique for decreasing the total collector resistance is to diffuse deeply the N+ collector connection region such that it extends down through the N epitaxial layer and into contact with the N+ buried layer region. Since the path between N+ buried layer and the collector contact region accounted for about forty percent of the total collector resistance, a substantial reduction in collector resistance is accomplished. However, the lateral path from under the base region over to the deep diffused collector contact region still remains and accounts for a large part of the total resistance.
In a discrete transistor, no such lateral path exists and the collector resistance is low. The emitter, base, and collector regions are arranged in vertical alignment, with the N epitaxial region, in which the base diffusion is made, being grown on an N+ substrate, and the under surface of the substrate is utilized as the collector connection surface. The principal resistance is formed only between the P base and the lower edge of the N epitaxial layer, and thus the collector resistance is low and such a discrete transistor has good current handling characteristics.